Question about FOX vTec + Multi

[PerFlare]

Hi,

I jump to rigs one with a vtec/multi in an one with a standard canopy in it. I have +400 jumps on the standard canopy and +200 jumps with the vtec/multi. I jump them often directly after each other from the same height from the same local 380ft antenna.

I have problems with offheadings with my vtec/multi canopy, I have sent it to BR for measurements but it come back okay. But I have ALLOT more severe offheadings with the vtec/multi and I noticed that it are usually when jumping in side wind. jumping in side wind usually turns your canopy up against the wind but anyway the vtec/multi are very, very sensitive.

the same is if you are not totally level .. bang and you have a offheading. and with arials and floaters you are often not 100% in level. I have even some problems just doing a normal exit, can feel it very early if I not in perfect balance ... it will open offheading.

I'd like to test if there are really the multi that are making the canopy so sensitive Is it possible to just use one attachment point and take the other three away just to test my theory? or at least take the one on the number 3 and the number 5 cell away and just using the two on the center cell.

If you do draw it up and look at the geometric of a canopy witch a multi on you can see that it will be more sensitive.

so question number one :
is it possible to just use one of the attachment point, or at least just the two on the center cell?

and question number two :
Anybody having the same experiance?

Thanks
PerFlare
http://www.linetwist.net

[imported_Tom Aiello]

Hi Perflare,

>is it possible to just use one of the attachment points

I have done this on several occasions. I believe that BR officially recommends it (in the owners manual) for static line deployments only. However, since the multi's major benefits occur at high airspeeds (near terminal), it ought to be ok to remove it for, say, any slider down jump.

I just removed the multi entirely and attached th bridle to the top center attachment (the red one). I had no problems with it.

>Anybody having the same experience?

No. In my experience, having the multi attached or not has had minimal effect on heading for subterminal jumps. I actually took it off to test the other way (would heading degrade without it), and found that it made no real difference.

I'd think that a more likely explanation for the phenomena you are observing (greater wind sensitivity, greater body position sensitivity) would be the Vtec, not the multi. Is your other canopy a Vtec?

Since the Vtec allows inflation through the bottom skin, it ought t increase susceptibility to cross winds during deployment. Here's why I think so:

As the canopy lifts off your back, it will describe an arc to one side (with the wind), rather than a straight line. Once it reaches line stretch, the bottom skin will be partially presented to the wind (more presented with more wind, and hence a greater angle from dead vertical). With an unvented canopy, this shouldn't matter too much, since the dominant opening inflation wil occur through the nose. However, with a vented canopy, inflation will also occur through the bottom skin vents, effectively initiating inflation from the side, before the canopy has the opportunity to "right" itself due to nose first inflation.

Anyone else have any ideas as to the wind susceptibility of vented canopies?

--Tom Aiello
tbaiello@mac.com

[BASE_689]

I am sorry to hear that, Per. I have two Prism's, both with a Fox Multi Vtec, and, talking about slider off jumps, the worst case I got is a 45° off-heading (just once). Yes, I don't have hundreds of jumps, but without slider I get every time quite consistent on heading openings.
Honestly, I cannot think about a simple reason that can cause, unless some involuntary indirect trapping of "non-tail-gate-lines" into tail gate, I cannot think about anything else.

> ...or at least take the one on the number 3 and the number 5 cell away and just using the two on the center cell...
Don't do it!!! DON'T DO IT!!! DON'T DO IT!!! I think it's an extremely unstable configuration, and furthermore it's not "covered" by BR as a possible configuration on Multi.
The only (safe) thing you can do is to hook your bridle onto the front ring on central cell, so riggin your canopy exactly as a "non-multi" Fox. Try and do some jump in this configuration, and see what happens.

Stay safe out there
Blue Skies and Soft Walls
BASE #689 :D

[JOMAN]

You could try using video you can slow this down and some times see what is causing the issue. This is something a friend i jump with does. It has worked well on some occasions to pinpoint a perticular problom. just a thought ;-)

JOMAN

[Micke N]

I have the same problem with the same configuration as PerFlare. When doing slider down jumps I need to have perfectly leveled shoulders to get on heading openings, if I'm not in level it opens in any direction. I have studied videos of my openings, jumping in low or zero winds, and I don't think the Vtec play any part in this problem. It's the packjob that get twisted, way before pressurisation. The canopy inflates nicely... in the direction that the packjob decide to turn to.

Micke N

[imported_Tom Aiello]

Micke and/or PerFlare,

Have you checked the continuity of the multi lines?

A continuity failure might create a repeated tendency to twist.

--Tom Aiello
tbaiello@mac.com

[guest]

>I don't think the Vtec play any
>part in this problem. It's the packjob that get
>twisted, way before pressurisation. The canopy
>inflates nicely... in the direction that the
>packjob decide to turn to.

I think Tom's original comment holds even at that. During opening, the jumper has a chance to come to rest, horizontally, with respect to the air he's moving through. A canopy that inflates through the bottomskin before having a chance to line itself up above the jumper that way might have a greater chance of opening off-heading in a crosswind.

For what it's worth, in my very limited experience (28 jumps, almost all VTec/Multi) off-headings have been greater than five degrees only three times, and with the exception of my first jump (ech) have always been less than fifteen degrees. The on-headings include a couple of jumps where I was sure I was going to be hosed.

[PerFlare]

Tom,

Thanks for your reply.

I will reconfigure and just use the top attachment point on the center cell. I rarely do long slider down delays so in my case it should be okay.

My other canopy are a standard canopy. With a single pilot chute attachment point. With this canopy I have very nice onheading frequency. Still in side winds it opens more "up wind" but no more then expected.

One "problem" are that I just own two base canopies. and on the newer one I ordered vTec and Multi so it is very hard to say if it is the Multi the vents or maybe the two things combined

Thanks again, and please forgive my poor English. But I guess not many of you speak Swedish.

SeeYa
PerFlare

[PerFlare]

BASE689,

Thanks for your reply.

I jump the FOX vTec + Multi in a Prism to. It is a very nice rig.

I can say with great deal of confidence that I'm not make any rigging errors with the tailgate. If I did it would be the same on both my canopies. I put the tailgate the same on both canopies.

Thanks for the warning about the attachment points I will as both you and Tom Aiello are recommending and just use the top attachment point on the center cell.

SeeYa
PerFlare

[PerFlare]

Tom,

Thanks again for your help.

Yes, you are right. When I first got the canopy and started packing I noticed that it did not feel the same as to one I already was jumping (the standard canopy). I can't really say what it is but I still have the same feeling. It is like the trim or the profile are a little different.

I did return the canopy to BR and they could not find any strange measurements on it. I like to say, when I did send my canopy away I did received a "replacement" canopy from BR and that's customer service! Thanks again BR.

So I have not checked the measurements my self but I trust BR's riggers to have checked that when I sent it back to re-measurement.

SeeYa
PerFlare

[guest]

Did you ask BR for their advice? They are the experts after all and have a lot of experience and get feedback from canopy users in the field. Contrary to what Tom says I must say, I thought the benefit of the multi was primarily for low air speed (slider off/down) jumps. It helps to prevent centre cell stripping and presents 3 cells to the relative wind for a better chance of on heading opening when it is more critical ie on lower jumps when you are closer to the object (no time to track). Most people I know take the multi off for slider down, longer delay jumps as it tends to make it whack open.:P

[PerFlare]

Pete,

Thanks for your post.

Yes I have tried to ask BR for their advice and asked them about what they hear from other jumpers and if it was possible to not use the multi. My first post actually are a copy of a mail to BR.

I'm very interested in this concept of choosing when to use the multi. Do your friends use the single mode even slider up, and slider up beyond 8 seconds?

Yes, my experience are also that it do open pretty hard and I will start using my sail slider on bigwall jumping.

Thanks
PerFlare
http://www.linetwist.net

[imported_Tom Aiello]

If the primary purpose of the multi is to avoid center cell strip, shouldn't it be most useful at terminal?

Center cell strip is a greater problem with greater snatch force. Snatch force is greatest at terminal (even assuming a 32" PC at terminal and a 48" at 2 seconds) because air speed is the greatest determiner of snatch force--far greater than PC size.

So, if air speed determines snatch force, and snatch force determines center cell strip, shouldn't greatest center cell strip (and hence maximum multi usefulness) occur at the point of maximum snatch force (i.e. terminal)?

That's basically the reasoning I heard from Nick (BASE 194) when I bought my first multi-equipped canopy, and he was working at BR. He also had some pretty good photos to support his argument.

Perhaps someone from BR can comment?

--Tom Aiello
tbaiello@mac.com

[PerFlare]

Tom,

Thanks for your input.

Yes I agree. About snatch force and center cell stripping. But I do disagree on one major point, center cell stripping as a major problem. I have more problems with the Multi then I have with center cell stripping on my single attachment point canopy.

This is of cause if that really are the Multi that makes my canopy so sensitive. I will try to jump it with only one attachment point and see if I are able to get a better onheading frequency

If I get it to open more onheading I will accept that my center cell stripping frequency goes up.

Thanks
PerFlare

[imported_Tom Aiello]

As far as I know, no one has ever done any actual research (you know, with control groups, monitored circumstances, and statistical analysis) on the effects of center cell strip.

I think it's one of those things that we've all seen on video and nodded our heads, and said "Yep, that's got to be bad."

I'd be really interested in hearing what your conclusions are after you start jumping without the multi.

Does anyone know if any statistics have been gathered on the actual effect of center cell strip on opening? Did anyone from BR gather this kind of info during the multi development?

As an aside, Per, you might want to ask Adam at CR about this. I know he has been working on an "anti-center cell strip" mechanism that hasn't yet made it into public. If you're doing jumps with and without the multi to test, he might be convinced to let you try his new system and give feedback on that.

--Tom Aiello
tbaiello@mac.com